The present invention relates to surface wave devices and especially to surface wave filters with proximity coupling used in many applications, especially in portable telephones.
Surface wave filters use the propagation of acoustic waves on the surface of a piezoelectric substrate, and the filtering characteristics are obtained from the modifications made in the propagation of these waves as a function of their frequency by electrodes appropriately arranged on the surface of the substrate. There are several structures for these electrodes, and a choice is made from among them as a function of the result to be obtained. These structures include, more particularly, structures using resonators transversely coupled by proximity.
The basic principle of a filter with transversely coupled resonators is described herein, for reasons of clarity, by means of a known example of a two-pole filter. A filter of this kind has two resonators each formed by a transducer between two reflector gratings and coupled by proximity. The transducers and reflector gratings usually have an interval of metallization equal to about half a wavelength. The first resonator is connected to the input of the filter and the second resonator to the output. The input resonator includes a comb-shaped electrode whose fingers are interdigitated with the fingers of a ground electrode. The acoustic waves thus generated by this transducer are emitted towards the right and left towards reflectors formed by fingers parallel to the fingers of the transducers and obtained by cutting out the ground electrode. The resonator connected to the output is symmetrical with the resonator connected to the input with respect to an axis parallel to the direction of propagation of the acoustic waves generated by the input resonator. It has a transducer and two reflectors. Since the ground is common, the corresponding ground electrode is all in one block and is connected to a ground output. The coupling of the two resonators is achieved by proximity, by bringing them together. Thus, it may be considered that the structure is approximately equivalent to the one obtained by bringing two waveguides together. Each of the modes of propagation of the guides considered alone is converted into two modes, a symmetrical mode and an antisymmetrical mode. When the distance between the two resonators is great, the speeds of the acoustic waves in these two modes are very close to each other and substantially equal to the speed of the mode of an isolated waveguide. The coupling is then very weak. This fact is all the more obvious as the resonators are at a distance. When the distance between the two resonators diminishes, the speed of the waves in the two modes diverges and the coupling increases. It can be shown that the equivalent diagram of a structure of this kind corresponds approximately to that obtained for two coupled resonators with a coupling equal to a standardized difference between the frequencies f.sub.s and f.sub.a of the two symmetrical and antisymmetrical modes given by the formula: ##EQU1##
This coupling thus defines the relative passband that can be obtained with this type of filter. In general, a two-pole filter gives an excessively low rate of rejection, as well as a steepness of the flanks of the passband that is far too low. It then becomes necessary to make filters with four to six poles or even more, quite simply by the cascade connection of several two-pole structures.
The drawback of this structure lies in the presence of a central bus used as a common point, this common point being connected to the ground. The characteristic of this bus thus prohibits the use of a differential type of connection. Now a connection of this type is becoming increasingly necessary because of the development of integrated circuits towards a differential structure.
A first way of obtaining a transversely coupled surface acoustic wave filter with a differential structure is known to those skilled in the art (U.S. Pat. No. 5,365,138). This is a transversely coupled two-pole filter with differential inputs and differential or non-differential output wherein the transducer of the resonator connected to the input has been replaced by two different transducers separated by a half-wavelength. The ground is still common and the corresponding electrode is all in one block. By contrast, each of the transducers of the resonator connected to the input is connected to one of the differential inputs.
A structure of this kind thus enables differential operation, at both input and output, by the separation into two parts of the transducer of the resonator connected to the output in the same way as the transducer of the resonator connected to the input. By contrast, this type of filter has a major drawback that lies in the high input impedance of the filter. Indeed, the input impedance of the filter is the sum of the input impedances of each of the transducers for, between the two differential inputs, they are perceived as being series-connected by means of the ground electrode. To overcome this problem, there is another configuration that has a lower input impedance than that of the previous filter. The idea of this filter consists in making direct use of the input transducer in differential mode. This means that the transducer is connected between the + and - differential inputs. A structure of this kind requires the separation of the central bus into two, firstly a ground bus forming part of the resonator connected to the output and secondly a bus connected to one of the differential inputs forming part of the resonator connected to the input.
A structure of this kind thus enables differential operation at input with a low input impedance equal to the impedance of the input transducer. By contrast it has a major drawback as regards differential operation. This drawback is the absence of symmetry between the + differential input and the - differential input.
To overcome this drawback, it is possible to set up a parallel connection of two filters according to the above structure to obtain a two-pole filter with differential inputs (ref.: B. Wall, W. du Hamel: "Balanced driven transversely coupled resonator filters", 1996 IEEE Ultrasonics symposium Proceedings). The symmetry between the + differential input and the + differential input is obtained by reversing the polarity of the transducers to compensate for the reversal of their + and - differential inputs.
This last-named filter structure thus enables differential operation at input with totally symmetrical + and - differential inputs and a very low input impedance equal to the parallel connection of the impedance values of each of the transducers. However, it requires the making of a substantial substrate width.
An aim of the present invention is to overcome the above-mentioned drawbacks.